Audio player devices, particularly those with headphones, attempt to optimize loudness within given maximum headroom constraints. Examples of such devices include smartphones, tablet computers and standalone GPS devices. For this purpose, a special form of compressor known as a “limiter” can be used.
As is well known to those of skill in the art, “headroom” is the amount by which the signal-handling capabilities of an audio system exceed a designated level known as Permitted Maxim Level (PML). Headroom can be thought of as a “safety zone” allowing transient audio peaks to exceed the PML without exceeding the signal capabilities of an audio system (e.g. by generating clipping and/or other artifacts).
FIG. 1 is a graph illustrating operating characteristics of prior art compressors where signal levels in decibels (dB) are shown along a horizontal axis and output levels in dB are shown along a vertical axis. When the input level is below a threshold level, the output level is the same as the input level. This is referred to as a 1:1 compression ratio for the compressor. Also illustrated are other compression ratios including 2:1, 4:1, and 00:1 above the threshold level. With all compression ratios greater than 1:1, the output signal level will be less than the input signal level when the input signal level exceeds the threshold. A limiter is a special case of a compressor, having a compression rate of almost infinity above the threshold, i.e. having a compression ratio approaching 00:1.
The historical use of limiters was to fit unknown input signals into a known audio headroom for analog or digital recording devices, radio transmitters, etc. In more recent years, “make-up gain” limiters or “maximizers” have been used to increase loudness while maintaining peak signals within a given headroom. The additional gain can be either applied before the limiter (which needs increased headroom to cover the larger signals), or the threshold can be set to a fraction of the headroom, followed by a gain stage of 1/fraction. The higher the make-up gain and the shorter the release time of the circuit the more loudness increases, but only at the expense of the generation of artifacts due to rapid gain changes.
FIG. 2 is a high-level block diagram of a prior art feed-forward, single-band limiter. In the event that the input signal exceeds the threshold, the system calculates the gain reduction needed for the output to stay at or below the threshold level. That is, the input signal is split between a forward path containing a gain element (attenuator), and a side chain, that measures the level of the signal, compares it against a threshold level, and calculates the attenuation needed for the output to stay at or below the threshold level. Limiters usually detect the rectified peak level of the signal, and use short attack times. A theoretically ideal attack time of zero avoids the output exceeding the threshold for even transient input signals.
Single-band limiters work adequately well within a relatively a narrow frequency range for the input signal. For example, single-band limiters can be adequate for an input signal derived from the playing of a single musical instrument. However, when there are multiple instruments being played (e.g. a drum, a violin, etc.) distortion can be generated, sometimes referred to as an audible effect or “artifact” of the system. To address this problem, multiband limiters have been developed which provide separate limiters for different frequency bands, e.g. for frequency bands which might primarily apply to different musical instruments.
With a multiband limiter the frequency band is split into two or more bands such that adding the bands back together recovers the original signal. This can be accomplished with “constant voltage” filters like Linkwitz-Riley or phase linear FIR filters, as will be appreciated by those of skill in the art. With such multiband limiters, each band is individually limited and the outputs from all of the band limiters are summed together to obtain the output signal. Since the sum of the individual band limiters can exceed the threshold another limiter is needed after the summing point of the individual band limiters.
In FIG. 3, a prior art multiband limiter includes a low pass filter FL, a bandpass filter FB and a high pass filter FH. Each of the filters is provided with its own limiter, namely limiter LL, limiter LM, and limiter LH, respectively. The outputs of the three limiters are summed together in a summer S before the output is applied to a final limiter LLMH, which provides the signal output.
The purpose of multiband limiters, such as the multiband limiter of FIG. 3, is to increase loudness while avoiding audible artifacts. However there are cases where artifacts produced by multiband limiters may exceed those produced by single-band limiters. For example, when a tone or musical instrument is reproduced at a crossover frequency (e.g. at a frequency at the interface between two adjacent bands) the audio signal may be simultaneously present in both bands, with the result that the make-up gain increases its level in both bands. For example, a signal at the transition between the low pass filter band and the bandpass filter band might be amplified within both bands, doubling its level. In a subsequent wideband limiter this twice-as-loud signal significantly attenuates the level of the remaining signal.
It will therefore be appreciated that, with a multiband limiter of the prior art, a problem occurs when an input tone is at a cross-over frequency which can produce, at the summing point, two full-scale, in-phase signals that can attenuate the outputs of other stage(s) to be attenuated, resulting in an artifact. According to typical audio frequency distributions, this effect is much more likely to happen at the lower crossover frequencies, e.g. between the low pass band and the bandpass band in the example of FIG. 3.
FIG. 4 further illustrates this problem. A first graph A shows a Band1 corresponding to the band of the low pass filter of FIG. 3, a Band2 corresponding to the bandpass filter of FIG. 3, and a Band3 corresponding to the high pass filter of FIG. 3. As described above, artifacts can occur when a signal falls between two bands, such as between Band1 and Band2. This situation can be seen in graph B. As also described above, an input signal (“tone”) at the interface between Band1 and Band2 (e.g. between the low pass filter band and the bandpass filter band) tends to create more artifacts than a signal falling between Band2 and Band3 because lower frequency tones are generally louder than higher frequency tones. Graph B also shows a high frequency signal falling only in Band 3.
As seen in graphs C and D of FIG. 4, when a signal falls between Band1 and Band2, it shows up in both the Band1 and the Band2 outputs, effectively doubling the level of that signal. Since the outputs of Band1, Band2 and Band3 (as seen in graph E) are summed (see graph F) and then limited, the output signal of graph G has Tone1 (corresponding to the lower frequency input signal) of significantly higher level than that of Tone2 (corresponding to the higher frequency input signal). The audio signal has therefore been distorted and an audio effect or artifact has been developed.
Another problem prior art multiband limiters is that the various frequency bands are limited equally which does not provide optimal sound pressure level (SPL) or “loudness” in audio applications. Also, prior art multiband limiters require n+1 limiters, where n is the number of frequency bands.
These and other limitations of the prior art will become apparent to those of skill in the art upon a reading of the following descriptions and a study of the several figures of the drawing.